The present invention relates to a new and distinct variety of plum tree (Prunus domestica L.) which is named xe2x80x98HoneySweetxe2x80x99, in particular to a plum tree having the plum pox virus (PPV) coat protein gene which imparts a high level of resistance to infection by PPV.
The new variety is especially attractive commercially due to its PPV resistance. Plum pox virus is the most serious virus disease of plum and other Prunus species and has spread throughout Europe and England and has been reported in India, Egypt and Chile. More recently, outbreaks of the disease have been reported in the eastern United States and Canada. Plum pox virus causes serious fruit loss and can cause tree death. In the Czech Republic alone, the number of plum trees has dropped from a high of 18 million to less than 5 million due to the presence of PPV in the orchards. Thus a tree having resistance to infection by the virus would be a highly desirable commercial product in the U.S.
The new variety was originated in vitro by Agrobacterium tumafaciens-mediated transformation of open pollinated seed of the xe2x80x98Bluebyrdxe2x80x99 (not patented) plum with the PPV coat protein gene at the Appalachian Fruit Research Station, Agricultural Research Service, U.S. Department of Agriculture in Kearneysville, W. Va. Transformation of hypocotyl slices from an open pollinated seed of xe2x80x98Bluebyrdxe2x80x99 was carried out, and transgenic plants containing the PPV coat protein gene were successfully generated from the hypocotyl slices. In addition to the PPV coat protein gene, the plants also contained genes for kanamycin resistance (NPTII) and xcex2-glucuronidase (GUS), transformation selection markers. The transformation and regeneration process is described in detail in Scorza et al. (1994. Plant Cell Reports. vol. 14, pp. 18-22).
The new variety was selected from a group of 21 clones derived from the transformation of hypocotyl slices from open pollinated seed of the xe2x80x98Bluebyrdxe2x80x99 plum. The new variety was distinct from the remaining transgenic clones containing the PPV coat protein, NPTII and GUS genes due to the presence of multiple copies of the PPV coat protein transgene, the low amount of PPV coat protein RNA produced and the absence of detectable PPV coat protein (Scorza et al., supra). Resistance is mediated through post transcriptional gene silencing (as described in Scorza et al. 2001. Transgenic Research. vol. 10, pp. 201-209).
While the female (seed) parent of xe2x80x98HoneySweetxe2x80x99 is xe2x80x98Bluebyrdxe2x80x99, the pollen source (male parent) is unknown. The new variety is distinct from its seed parent by its fruit quality, earlier ripening date (about 1 week earlier) and by the presence of the PPV coat protein, NPTII and GUS transgenes and by its high level of resistance to PPV. Since, at the time of the invention, no other plum tree contained these genes either singly or jointly, the pollen parent could not have contributed any of them, thus the new variety is also distinct from its pollen parent. Plum pox virus resistance is described in Ravelonandro et al. (1997. Plant Disease. vol. 81, pp. 1231-1235).
The new variety was originally selected in vitro as a regenerated shoot from a xe2x80x98Bluebyrdxe2x80x99 plum seed hypocotyl slice that had been transfected with Agrobacterium tumefaciens EHA 101 carrying the plasmid pGA482GG/PPV-CP-33. The regenerated transformed shoot was rooted in vitro and transferred to the greenhouse for continued growth. After two years in the greenhouse, the original greenhouse-grown plant was field planted and maintained under close observation.
The new variety was asexually propagated (originally from the greenhouse-grown plant) by bud-grafting on to standard rootstocks, including but not limited to Prunus persica (GF305 peach), Prunus domestica (European plum seedlings), Prunus myrobalan and [Prunus cerasiferaxc3x97P. munsoniana] (GF 8-1). Comparisons of asexually propagated trees and the original plant of the new variety have shown that the characteristics of high level of PPV resistance, vigorous growth, upright tree form, productivity, high fruit quality and large fruit size are maintained. No aberrant types appeared.
The new variety serves as an effective parent for transferring the PPV coat protein gene and resistance to PPV (as described in Scorza et al. 1998. Acta Hort. vol. 472, pp. 421-427 and Ravelonandro et al. 1998. Acta Hort. vol. 478, pp. 67-71). The transgene insert is transferred as a single genetic locus and resistance acts in a dominant manner. This simply inherited dominant resistance provides resistance as described above to the major known serotypes of PPV (Ravelonandro et al. 2001. Acta Hort. vol. 550, pp. 431-435). The plant is not self-fertile; a pollinator is required.
Transformation of plum has been described in the literature, however trees having the characteristics as described herein have not been produced. Researchers have been unable to achieve the same combination of fruit quality, resistance to PPV, productivity and tree growth that makes the new variety commercially attractive for fruit production.
The new and distinct variety of plum tree is large, productive, vigorous with an upright growth habit. The fruit is medium to large in size, is freestone with slight cling and is of excellent eating quality. The fruit has yellow flesh of firm texture and sweet flavor and blue skin. The new variety is distinguished from all other plum trees by resistance to plum pox virus infection due to the presence of the plum pox virus coat protein gene.